1. Field of the Invention
The present invention relates to a method of exposure employing phase shift mask of attenuation type and, more particularly, to a method of exposure for exposing a material having a stepped portion.
2. Description of the Background Art
Recently, semiconductor integrated circuits have been remarkably improved in the degree of integration and miniaturization. Accordingly, circuit patterns formed on the semiconductor substrate has been rapidly reduced in size. Photolithography is widely recognized as basic technique in patterning. Though various developments and improvements have been made to date, there is stronger demand in improved resolution of patterns, as patterns have come to be smaller and smaller.
The resolution limit R (nm) in photolithography employing the magnification exposure method is represented as EQU R=K.sub.1 .cndot..lambda.(NA) (1)
where .lambda. represents wavelength (nm) of the light beam used, NA represents numerical aperture of the lens, and K.sub.1 is a constant dependent on the resist process.
As can be seen from equation (1), in order to improve resolution limit, the values of K.sub.1 and .lambda. should be reduced while the value of NA should be increased. In other words, the constant dependent on the resist process should be made smaller while the wavelength of the light beam should be reduced and the numerical aperture should be enlarged. However, improvement in the light source and the lens is technically difficult and, in addition, if the wavelength is made shorter and NA is increased, depth of focus .delta. (.delta.=K.sub.2 .cndot..lambda./(NA).sup.2) becomes shallower, resulting in undesirable lowering of the resolution.
Referring to FIGS. 11(a), (b) and (c), cross section of a photomask, electric field of the exposure light on the photomask and intensity of the exposure light on the semiconductor wafer will be described.
First, referring to FIG. 11(a), cross sectional structure of a photomask 100 will be described. On a quartz glass substrate 110, a metal mask pattern 120 formed of chromium or the like and a light transmitting pattern 130 exposing the quartz glass substrate 110 are formed. Referring to FIG. 11(b), the electric field of the exposure light immediately after the passage through photomask 100 will be described. The electric field of the exposure light on photomask 100 is conforming to the photomask pattern. Referring to FIG. 11(c), light intensity on the semiconductor wafer will be described. As to the intensity of exposure light on the semiconductor wafer, especially when fine pattern is to be transferred, the beams of exposure light which have passed through the photomask are overlapped at adjacent pattern images because of diffraction and interference of light as shown in the figure, and thus the intensity is increased. As a result, the difference in the intensity of light on the semiconductor wafer becomes smaller, resulting in poor resolution.
In order to solve this problem, Japanese Patent Laying-Open No. 57-62052 and 58-173744 proposed a phase shift exposure method employing a phase shift mask. Referring to (a), (b) and (c) of FIG. 12, the phase shift exposure method using a phase shift mask disclosed in Japanese Patent Laying-Open No. 58-173744 will be described. Referring to FIG. 12(a), the cross sectional structure of phase shift mask 200 will be described. The phase shift mask 200 includes a chromium mask pattern 220 formed on a glass substrate 210, and a light transmitting portion 230 at which glass substrate 210 is exposed. Further, at every other light transmitting portions 230, a phase shift 240 formed of a transparent insulating film such as a silicon oxide film, is provided.
Referring to FIG. 12(b), the electric field of the exposure light on the phase shift mask will be described. The electric field of the exposure light on the phase shift mask provided by the exposure light which has passed through the phase shift mask 200 has its phase inverted by 180.degree. alternately. Therefore, the light beam offset with each other at adjacent pattern images where exposure light beams overlap with each other, because of light interference. The intensity of exposure light on the semiconductor wafer will be described with reference to FIG. 12(c). As shown in the figure, the difference in intensity of exposure light on the semiconductor wafer is distinctive, and thus resolution of the pattern image can be improved.
The phase shift exposure method employing the phase shift mask is very effective for periodic patterns having lines and spaces, for example. However, if the pattern is complicated, positioning of the phase shift is very troublesome, and therefore this method cannot be applied to every desired pattern.
As a phase shift mask solving this problem, an attenuation type phase shift mask is disclosed, for example, in JJAP Series 5 Proc. of 1991 Intern. Micro Process Conference pp. 3-9 and in Japanese Patent Laying-Open No. 4-136854. The phase shift mask of attenuation type disclosed in Japanese Patent Laying-Open No. 136854 will be described in the following.
Referring to FIG. 13(a), the structure of the attenuation type phase shift mask 300 will be described. The attenuation type phase shift mask 300 includes a phase shift pattern, which is a prescribed exposure pattern, that includes a quartz substrate 310 transmitting exposure light, light transmitting portions 330 formed on the main surface of the quartz substrate 310 and exposing the main surface of the quartz substrate 310, and a phase shift film 320 for shifting the phase of the exposure light passing therethrough by 180.degree. with respect to the phase of the exposure light passing through said transmitting portions 330. The phase shift film 320 has a two-layered structure consisting of a chromium layer 320a having the light transmittance of 5 to 20%, and a shift layer 320b providing phase difference of 180.degree. between the light passing therethrough and the light passing through the light transmitting portions 330. Recently, chromium oxide, chromium nitride oxide, chromium nitride carbide oxide, molybdenum silicide oxide, and molybdenum silicide nitride oxide have come to be used as a one layered structure phase shift portion, instead of the aforementioned phase shifter portion 320.
Provision of the attenuation type phase shift mask allows exposure of even a complicated pattern, and therefore this has become dominant in the recent exposure methods.
However, semiconductor devices have very complicated cross sectional structures these days. Therefore, when a resist film is to be exposed by using the aforementioned attenuation type phase shift mask, there is often a step in the material underlying a resist film. Therefore, optimal focal position may differ from one position to the other, even in one and the same step of exposure.
For example, let us consider an example in which there is a step in an underlying substrate 12, a resist film 14 is formed on the substrate 12 having the step and resist film 14 is to be exposed, with reference to FIGS. 14 to 16.
First, referring to FIG. 14, assume that the focal position is at the left side region of resist film 14. In this case, at the left side region of resist film 14, a desired pattern 14a can be formed. However, in the right side region of resist film 14, a pattern 14b is formed with the diameter being smaller than desired.
Referring to FIG. 15, let us assume that the focal position is in the right side region of resist film 14. In this case, a desired pattern 14b can be formed in the right side region of resist film 14. However, in the left side of resist film 14, a pattern 14a is formed with its diameter made larger than desired.
Referring to FIG. 16, assume that the focal position is at the intermediate portion of the step. In this case, in the left side region of resist film 14, pattern 14a is formed with its diameter being larger than desired, while in the right side region of resist film 14, pattern 14b is formed with its diameter made smaller than desired.
As described above, when there is a step in the resist film, optimal focal position cannot be obtained for every region in one and same step of exposure, and hence desired pattern cannot be formed in the resist film.